Conventionally, as electrically driven rotating tools such as electric drivers, etc., which are driven by electric motors, tools have been proposed and worked which are constructed so that in cases where a strong counter-load is applied to the driver bit during the tightening of screws, etc., a state in which a specified torque value is reached is detected by a clutch mechanism which operates at a preset tightening torque, and the clutch mechanism operates to temporarily sever the coupling between the output shaft of the electric motor and the driven shaft (driver bit). Furthermore, electric drivers, etc., which are constructed so that when the clutch mechanism operates, this state is detected by a limit switch, etc., and the driving of the electric motor is stopped, have also seen practical application (Japanese Patent Application Publication (Kokoku) No. 60-13798).
More specifically, electric drivers equipped with such a clutch mechanism includes a structure comprised of an automatic clutch device in which, for example, the output shaft of the electric motor is coupled to the driver bit via a planetary gear speed reduction mechanism, an internal gear which engages with the planetary gears of this planetary gear speed reduction mechanism is loosely fit inside a grip portion casing so that this internal gear can rotate, this internal gear and one end of the grip portion casing are respectively closed off and caused to face each other, a through-hole is formed in the facing surface of the grip portion casing, a steel ball is accommodated in this through-hole, and this steel ball is elastically held by a flange-equipped sleeve from the outside of the grip portion casing so that the steel ball is inserted into and caused to contact the inside of a cam groove formed in the facing surface of the internal gear.
Furthermore, in the above-described automatic clutch device, the rotational output from the output shaft of the electric motor is transmitted to the driver bit via the planetary gear speed-reduction mechanism in the tightening work of screws or the like; and as the completion of such tightening of a screw is approached, a counter-load is transmitted to the planetary gear speed-reduction mechanism from the driver bit, and this acts to cause rotation of the internal gear via the planetary gears. Accordingly, when this counter-load overcomes the elastic force that is pressing the steel ball, in other words, when this load exceeds a specified set torque, the internal gear rotates so that the steel ball rides over the cam groove formed in the facing surface of the internal gear; as a result, the coupling between the output shaft of the electric motor and the driver bit is temporarily severed. Accordingly, it is possible to alter the engagement point of the clutch device, i.e., the set value of the torque, by adjusting the elastic force of the flange-equipped sleeve that holds the steel ball.
An automatic power cut-off device for electrically driven rotating tools which is devised so that the driving of the electric motor can be simply stopped by using a combination of a magnet piece and an magnetic detection element (Hall element) as means for detecting the actuation of the clutch mechanism, and constructing a circuit that cuts off the power to the electric motor, which includes the magnetic detection element, is also proposed (Japanese Patent Application Publication (Kokoku) No. 60-3960).
Furthermore, in electrically driven rotating tools of this type, an external AC power supply (commercial power supply) is generally used for driving control of the electric motor; in such cases, a control unit which has an AC/DC power conversion function and a torque control function, etc. is used in order to obtain a power supply output suitable for the driving of an electric motor from such an external AC power supply. In cases where an ordinary small DC motor is used as the electric motor, this control unit is constructed as a unit that is independent of the electrically driven rotating tool, and driving control of the electric motor is accomplished by connecting this control unit between the AC power supply and the electrically driven rotating tool.
Nowadays, furthermore, in regard to DC motors, the use of brushless motors which are superior in terms of characteristics such as non-contact operation, prevention of noise, high torque and small size, high-speed rotation and long useful life, etc., and which offer the advantage of being maintenance-free, as electric motors in electrically driven rotating tools, has been proposed; and such motors have seen practical use. In the driving control of such brushless motors, unlike the case of the DC motors, a driving circuit that generates a rotating magnetic field is required. Furthermore, such a driving circuit can be constructed by means of a magnetic pole sensor which detects the positions of the magnetic poles of the magnet rotor (generally, a Hall element is used), a driving coil which is excited in accordance with the rotor magnetic pole positions so that a rotational force in a fixed direction is applied, and a special IC circuit which powers and controls the magnetic pole sensor and driving coil.
The driving circuit constructed in this manner can be accommodated inside the grip portion casing of the electrically driven rotating tool as a compact circuit structure together with a circuit which has a torque control function, etc. Accordingly, in cases where a brushless motor is used, a control unit having a structure that is independent of the electrically driven rotating tool is unnecessary; only an AC/DC converter is required, and the driving circuit, etc., can be contained in the electrically driven rotating tool and built with a simple structure, so that handling can be simplified.
In a conventional electrically driven rotating tool, as described above, micro-switches, limit switches, etc. are used in the torque detection mechanism, etc., beginning with the driving switch that initiates the driving of the electric motor; accordingly, sparks, etc. are generated at the switch contact points during the operation of the switches, and this leads not only to wear of the contact points, but also various problems with respect to the peripheral electronic parts, electronic devices and electronic circuits, etc. Accordingly, in the case of such mechanical switching mechanisms, there are structural limits to how far compactness and an increase in the useful life can be achieved, and the degree to which the electrically driven rotating tool as a whole can be made more compact is also subject to many restrictions in terms of structure and disposition.
The inventor of the present application conducted diligent research and the structure of prototypes in order to solve such problems. As a result, the inventor discovered that the power circuit, etc. can also be made extremely small and compact by using combinations of magnets and magnetic sensors as switches, such as the driving switch and torque detection mechanism, etc., that are accommodated inside the grip portion casing of an electrically driven rotating tool. In particular, the inventor ascertained that in cases where a brushless motor is used as the electric motor, the driving switch and the like can all be compactly accommodated inside the grip portion casing of the electrically driven rotating tool along with the driving control circuit of the electric motor, so that handling can be simplified.
However, in cases where a brushless motor is used as the electric motor, in the control of the torque, the operating characteristics of the motor, especially in regard to the rotation of the rotor, are set so that the inertial moment is extremely small as in a stepping motor; accordingly, the system has characteristics in that in the case of stop control of the brushless motor, the rotor immediately stops rotating without any inertial force. Consequently, in cases where torque control is performed by installing a conventional clutch mechanism, some difficulties are encountered. Namely, for example, as the completion of the tightening of the screw is approached, a counter-load is transmitted to the planetary gear speed reduction mechanism from the driver bit; and when the torque detection mechanism performs a detection and stops the driving of the motor at the clutch engagement point at which this counter-load overcomes the elastic force that is pressing the steel ball, so that the torque exceeds the specified set torque, thus causing the internal gear to rotate so that the steel ball rides over the cam groove formed in the facing surface of the internal gear, then the driving power supply of the motor is cut off without the steel ball completely riding over the cam groove; as a result, the driver bit rotates back so that the tightening of the screw at the specified torque cannot be completed.
Accordingly, the present inventor conducted further diligent research and investigations. As a result, the inventor discovered that the above-described problems can be completely solved by constructing an electrically driven rotating tool so that a driven shaft is coupled as an working shaft to the output shaft of an electric motor via a speed-reduction mechanism, a clutch mechanism equipped with a cam engaging section which operates so that the engagement between the output shaft and the driven shaft is cut of when a load torque that exceeds a predetermined value acts on the driven shaft is installed, a torque setting mechanism in which the engagement point of the clutch mechanism is adjusted as a set torque value is provided, and a torque detection mechanism which performs a drive stop control of the electric motor at the same time that the actuation of the clutch mechanism is detected; and in this electrically driven rotating tool, the torque detection mechanism is set so that a drive stop control of the electric motor is performed at the same time as the detection operation in a state in which the cam engagement in the cam engaging section of the clutch mechanism is completely released and the clutch operation is completed.
Furthermore, in cases where a torque setting mechanism is installed in an electrically driven rotating tool equipped with the above-described torque detection mechanism, a structure in which the torque setting mechanism is concentric with the driven shaft in correspondence with the clutch mechanism is taken. As a result, there is frictional contact between the torque setting mechanism and the driven shaft, and the precision of the torque control effected by the torque setting mechanism with respect to the clutch mechanism tends to drop. In view of this, the inventor discovered that by way of using an independent structure in which the torque setting mechanism that is disposed so as to face the clutch mechanism is provided at an inclination so that the torque setting mechanism is not concentric with the driven shaft, it is possible that an adjustment of the torque by means of the torque setting mechanism is simply performed at any time without removing the driver bit as in a conventional system, and the precision of torque control such as torque setting and torque detection, etc. is conspicuously improved.
Accordingly, the object of the present invention is to provide a torque control system for an electrically driven rotating tool that is equipped with a clutch mechanism that cuts off the engagement between the output shaft and the driven shaft when a load torque that exceeds a predetermined value acts on the driven shaft; and in the system of the present invention, a drive stop control of the electric motor is performed at the same time as the detection operation by a magnetic sensor when a state is reached in which the clutch operation of the clutch mechanism is completed, thus making it possible for the constant-torque tightening of screws, etc. to be performed appropriately and with good efficiency at all times, and also making it possible to achieve easily an increased compactness of the device as a whole and an improved torque control precision.